Ionic liquids, also know as molten salts, are made up of salts derived from tetra alkyl ammonium or phosphonium or, more frequently, made up of heteroaromatic cations associated with anions, such as, for example, BF4, PF6, CF3SO3, (CF3SO2)2N, CF3CO2 (P. Wasserscheid, T, Welton; Ionic Liquids in Synthesis, VCH-Wiley, Weinheim, 2002; J. Dupont; R. F. de Souza, P. A. Z. Suarez; Chem. Rev.; 2002, 102, 3667; P. Wasserscheid, W. Keim; Angew. Chem. Int. Ed.; 2000, 39, 3773; T. Welton; Chem. Rev.; 1999, 99, 2071), and in a general way these ionic liquids are mainly used industrially as reagents or solvents.
The most researched and used ionic liquids are those based on the 1,3-dialkylimidazolium cation, and its physical and chemical properties qualify it as a “green” solvent in many processes, such as, for example, processes of extraction/separation, synthesis, catalysis, electrochemical.
The use of ionic liquids as a “green” reaction medium is primordially described as substituent for conventional mediums in chemical processes.
With growing concerns about the environment, the use of ionic liquids as a reaction medium can provide a way to minimize the production of wastes. In ionic liquids it is possible not only to efficiently promote reactions, but also to contribute significantly to minimize solvent loss.
There are applications in which the ionic liquids play the role of lubricating agent between metallic parts that undergo a high level of mechanical wear. Again, the absence of free halogens that may form cells in the presence of small amounts of water or polar compounds is very important. It is known today, that in industry, minimal amounts (mg/L) of halogens compounds in pyrolysis furnaces feedstocks, for example, can lead to planned maintenance down time due to corrosion in the pipes or even disintegration of refractories.
Therefore, the use of ionic liquids in addition to providing ecological benefits, also translates into economic advantages.
The Article by J. S. Wilkes et al (Inorg. Chem.; 1982, 21, 1263) presents a synthesis of 1,3-dialkylimidazolium chlorides that makes it possible to introduce similar or different alkyl groups. Mixtures of these chlorides with anhydrous aluminum chloride, in various proportions, provide ionic liquids.
Another Article by J. S. Wilkes et al (J. Chem. Soc., Chem. Commun.; 1992, 965), explains a method for exchanging a chloride salt of 1,3-dialkylimidazolium ion with various anions, such as BF4 and CH3CO2, by reacting imidazolium chlorides with a silver salt containing the desirable anion.
The Article by J. Dupont et al (Polyhedron; 1996, 15, 1217) describes a new method for this reaction, with a sodium salt used as the desired counter ion and acetone as solvent.
The Article by J. Dupont et al (Org. Synth.; 2002, 79, 236) presents a detailed optimization of the experimental procedure of replacing a halogen anion of the 1,3-dialkylimidazolium salts with BF4, PF6 or CF3SO3.
The patent belonging to P. Wasserscheid et al (EP 03/02127, dated Sep. 12, 2003) describes the synthesis, through metathesis reactions, of some ionic liquids with a general formula of [cation]+.[ROSO3]—. Thus, for example, the heating under vacuum of a mixture of 1-butyl-3-methylimidazolium with pyridinium diethylene glycol-monomethyl-ether-sulfate provides, after removing the pyridinium chloride by sublimation, the ionic liquid, butylmethylimidazolium diethylene glycol-monomethyl-ether-sulfate. In another procedure, a 1-butyl-3-methylimidazolium chloride interacts with ammonium diethylene glycol-monomethyl-ether-sulfate in CH2Cl2, the ammonium chloride precipitate was filtered and the filtrate was concentrated, yielding the ionic liquid, 1-butyl-3-methylimidazolium diethylene glycol-monomethyl-ether-sulfate.
The halogen metathesis method is well established nowadays; it allows synthetized, in a convenient manner, a wide range of ionic liquids derived from the cation 1,3-dialkylimidazolium. The residual contaminant is usually chloride that may be detected by testing with AgNO3 (1.4 mg/L limit), ionic chromatography (under 8 mg/L, in accordance with C. Villangran et al; Anal. Chem.; 2004, in press), or by cyclic voltammetry (ppb, according to B. K. Sweeny et al; Electrochem. Commun.; 2001, 3, 712). The water content may be determined by Karl-Fischer titration or by cyclic voltammetry (V. Gallo et al; J. Chem. Soc., Dalton Trans.; 2002, 4339). The determination of the presence and quantity of these impurities is essential in many applications, because the physico-chemical properties of the ionic liquids may vary significantly, depending on the water or halogen content (K. R. Seddon et al; Pure Appl. Chem.; 2000, 72, 2275).
Some processes for obtaining halogen free ionic liquids are described in the literature.
In K. R. Seddon et al's patent (WO 01/40146, dated Jul. 6, 2001) a process is described where the salts of 1,3-dialkylimidazoliums are prepared by alkylation of 1-alkylimidazolium with trifluoroethyl acetate or with butyl methanesulfonate, under reflux and purification by vacuum and heat, followed by a metathesis reaction of the anions with acids, such as, for example, HBF4 or HPF6.
In the Article by J. D. Holbrey et al (Green Chem.; 2002, 4, 407), 1-alkylimidazoliums are alkylated with dimethyl sulfate or with diethyl sulfate and, consequently, the anion (CH3OSO3 or CH3CH2OSO3) is exchanged for BF4, PF6 or CF3SO3.
The Article by K. Mikami et al (Tetrahedron Lett; 2004, 45, 4429) describes obtaining a salt of 1,3-dialkylimidazolium chiral through alkylation of 1-methylimidazolium with the triflic ester derived from (S)-ethyl-lactate (Diagram 1).

The salt shown above is a solid one, however, metathesis with PF6 allows a derivative of an ionic liquid to be obtained.
The Article by J. Dupont et al (Adv. Synth. Catal.; 2002, 344, 153), proposes a reaction where five components (glyoxal, formaldehyde, two different amines and an acid) are condensed to 1,3-dialkylimidazolium salts.
Undoubtedly, the derivatives of the cation 1,3-dialkylimidazolium associated with several anions are among the most investigated types of ionic liquids.
Very probably this is due to their facility to be synthesized, they are stable, and their physico-chemical properties can be fine-tuned by simply selecting the N-alkyl substituents and/or anions.
The great majority of these ionic liquids are usually prepared through the simple N-alkylation of N-alkylimidazol, generally using alkyl halogens as alkylation agents, followed by the association of metal halides or anion metathesis.
The anion metathesis procedures generate a great variety of ionic liquids based on 1,3-dialkylimidazolium of good quality.
Determining the purity of these ionic liquids is not a simple task. The principal contaminant is usually a residual halogen from the alkylation of imidazolium that may be detected by testing with AgNO3 (1.4 mg/L limit), ionic chromatography (under 8 mg/L), or by cyclic voltammetry (ppb). The water content may be determined by Karl-Fischer titration or by cyclic voltammetry. The determination of the presence and the quantity of these impurities is essential in many applications, such as in catalysis and spectroscopic investigation, once the physico-chemical properties of the ionic liquids may vary significantly, depending on the water and/or halogen content.
At all events, as mention before, according to J. Dupont, et al, ionic liquids 1,3-dialkylimidazolium halogen free may be prepared from the reaction of five components (glyoxal, formaldehyde, two different amines and acids) and those containing alkyl sulfate or trifluoromethane sulfonate anions by the simple alkylation of 1-alkylimidazolium with the corresponding dialkyl sulfate or an alkyl trifluoromethane sulfonate ester, respectively.
Among the advantages of ionic liquids based on 1,3-dialkylimidazolium cations we can point out the following:                They are non-volatile, with no measurable vapor pressure;        They are usually liquids within a wide range of temperatures (close to room temperature) and their viscosity is sufficiently low (<800 cP to 20° C.);        They have thermal and electrochemical stability more suitable than the usual solvents;        They dissolve a wide range of organic and inorganic compounds, on which their solubility may be adjusted by the choice of alkyl groups linked to the imidazole ring or by the nature of the anion;        They are typically non-coordinate solvents;        They are easily prepared from commercial reagents and through classic synthetic procedures.        
Similar procedures to obtain ionic liquids which use alkyl sulfonates and alkyl phosphate as alkylation agents have been patented. However, in almost all the work carried out in this area it has been observed that there is a strong participation of halogenated materials, and no matter what future application in industrial units industries might be for these ionic liquids, it will be very important to guarantee the stability of these materials and preferably the absence of these anions in their free form.
Currently, ionic liquids such as [butylmethylimidazolium] PF6, [butylmethylimidazolium] BF4 e [butylmethylimidazolium] (CF3SO2)2N are commercially available, but with relatively high levels of chloride contaminants.
However, it is surprising that up to now there is no quick method available to prepare and to determine the purity of 1,3-dialkylimidazolium cation halogen free associated with the most popular and the most used anions such as PF6, BF4 and (CF3SO2)2N.
It is clear that there is a need for simpler and more practical methods to prepare halogen free ionic liquids and also there is a need for a quicker and more direct methodology to determine their purities.